and this http://www.pwcdoctor.net/VERSIPLUG.html is imo the better option to a duckbill, it can't come off if u scrape it one something, footholds, turf and straightbars are a must to do anything freestyle or to learn at last
Any ski shop, like blowsion, wamiltons, JSU, R&D may have them. It works like a scupper but it's a rubber flap that lets water out and not in. You won't have to worry about sinking the thing from subbing to much.